The kinetics of kidney tubule induction were examined in transfilter experiments by varying the time of transfilter apposition, the porosity and pore size of the filters, and the culture conditions. Transfilter contact between the interacting cells is established within an hour when cytoplasmic processes emerge through the interposed filter; then a further 16-24 h are needed for completion of induction. This lag is a function of thickness and pore size of the filter, and is not reduced by precultivation of the inductor on the filter. Material that accumulates on the far side of the filter during such cultivation displays no morphogenetic activity. The intensity of the mesenchymal response was roughly quantified and shown to be a function of pore size, pore density and duration of transfilter contact.